Best friend.

It was… a concept. Rather like string theory, or indoor plumbing, or monetary policy—peculiar and useful, but not very immediate to Sherlock, at least not as a two-way thing. As a one-way condition, yes. If asked dispassionately, he'd have dispassionately stated that John Watson was his best friend. It was a fairly simple algorithm: a man with only one intimate friend has only one option for "best" friend, after all. Yes, all right, he'd been forced to concede that Mrs. Hudson and Lestrade and Molly were friends, too. But they didn't even know it, nor had he seen fit to officially inform them, which seemed to disqualify them as "best" friends in some way he didn't know how to specify, except that if they didn't know they were his friends they'd be ill prepared to then manage any of the duties of being best friends—of which, when he thought about it, the primary duty was knowing you were friends.

That did seem to be the core of it. Wasn't it? Your best friend is the person who knows he's your friend, and lives with it, even if he might rather not—an obligation John fulfilled quite well, especially considering everything.

Sherlock, pondering, had to conclude he didn't meet the obligations half so well. To begin with, he'd barely understood he was John's friend. John was his friend, but he'd really not been quite sure it ran the other way these days. And even in the days before his exile, he had to admit his awareness of John's return of friendship was flawed. It had to be flawed, as Sherlock found he got it wrong so often.

He was, in fact, a rather bad friend, as nearly as he could determine. A bit not good. A lot rather awful. A total prat, if he really forced himself to sit down and logic it through to the inevitable conclusions. ("Balance of probability," Mycroft snipped from the High Seat of Justice. "Preponderance of the evidence suggests…")

If Sherlock were to get married—and he couldn't imagine ever doing so, but just as a hypothetical—he'd have only one person to ask to be best man. For that matter, thinking about it, he'd have only eight people total he could imagine inviting. Well, nine, assuming a bride in the first place. And, he supposed, a lot more she might herself choose to invite, though he might get lucky and find someone like Mary with a relatively small social set. Which was getting quite off track, because this was a hypothetical marriage and he was now counting friends and relations on the hypothetical bride's side.

That road lay madness. He was sure of it.

In any case, there were only eight people he could imagine inviting. Mummy and Father, because, well—Mummy would make life impossible if he didn't invite them, after all. Mycroft because Mycroft would refuse anyway, and then Mummy would make him come whether he wanted to or not, and Sherlock would have the double pleasure of knowing his brother was there and knowing Mummy had forced him to come and to behave. And anyway, Mycroft, no matter how he grumbled, would wear the finest suit in the chapel barring Sherlock's own, which Mycroft would help pick out with much clucking and hissing and tutting—but Sherlock would know it was just right, because Mycroft would not ever, ever let Sherlock appear at such an occasion less than perfectly turned out. Then, after Mycroft, John, of course. Then Mary, because if John then Mary had to be there—and he liked Mary. Then Mrs. Hudson, and Molly, and Lestrade—and then Sherlock was out of options.

Of that list, well, Mummy and Father really didn't count. And one couldn't possibly ask Mycroft to be best man…not that he wouldn't do it splendidly, but Sherlock would then have to deal with him being much too splendid and smug, which would lead to a fight during the reception because that was the only thing Sherlock knew to do about Mycroft being all splendid and smug. Then the hypothetical bride would cry not-so-hypothetical tears, and Mummy would scold and Father would look tired and amused at the same time, as he so often did, and John would tell him it was all a bit not good, and Mary would stuff a napkin in her mouth as she tried not to laugh. Molly would be crying anyway, because…

No. Not going there. Delete, Sherlock. Delete that line of thought.

Lestrade would probably be taking photos and laughing his arse off. Which was why Sherlock couldn't ask Lestrade to be best man. Even if he'd been Sherlock's first pick—and John was Sherlock's first pick—but even if he'd picked Lestrade, Lestrade would start pulling out old photos and videos and explaining that if Sherlock made him wear a monkey suit and write a best man speech, he, Lestrade, would release all those photos and tapes to the public.

Which, given some of the footage Lestrade had in safe keeping, ended the conversation rather quickly, didn't it? Images going back close to a decade, now. If Lestrade and Mycroft had not been thick as thieves already, Sherlock would suggest to Mycroft that Lestrade ought to go on Mycroft's "people to be watched" list, right before Irene and right after Magnussen.

How had he got here from there?

Best man. Out of eight people—nine counting that bride he didn't have—there was only one he could ask to be best man. Even if Lestrade was a man, and rather good at it, insofar as Sherlock had ever been able to determine. A good man. A very good man. But…

Not Sherlock's best man.

It all made sense run in that direction, from Sherlock to John. All the pointy arrows on the chart aligned, logically, and there was no question or shadow of a doubt. Three relatives, one friend-in-law (pending John and Mary's ceremony, of course), and three friends, two of whom were disqualified by gender and the other who was both male and good—possibly even better than John in certain ways, assuming you could accept that anyone who worked with Mycroft wasn't automatically suspect—but who wasn't John. Logically Sherlock had one best friend, easily sorted from the short and highly limited heap.

John, though: people like John. Well, when they didn't detest him for being so reliably John-ish. Maybe it was more important to say John for the most part liked people, and they all tended to show willing and like him back on one of those baffling reciprocity deals people seemed to make behind Sherlock's back, without providing footnotes or annotations to clarify how it all worked. So in spite of the suppressed loathing thing, John had friends. Quite a lot of them. Scads. Heaps, even. Friends from med school, like Mike Stanford. Friends from the army. Friends he just seemed to make standing at bus stops carrying the milk and eggs home from Tesco's. Picking the best of them?

How had John even gone about it? He wasn't the sort to develop a spread sheet, though Sherlock thought he might better have done it that way, given the array he had to deal with and his limited mental skills. If a man with such a lack of memory retention was going to try to make a reasoned selection of best man starting with a lineup that had to be approaching at least thirty or forty candidates, if not more, he might really appreciate the compensatory benefits of good software.

After all, not everyone could be like Sherlock and Mycroft.

Oddly, the ones who were like him and Mycroft lacked the list of friends to justify the software in any case. And the ones who needed the software didn't have the brains or the software that would make up the difference.

Yet another puzzling thing to meditate on.

And still not answering the core question: how had John come to pick him as best man? Him—Sherlock. How?

There was, of course, the possibility of severe mental breakdown of some sort. Trauma induced by his absence and sudden return. Or just as possibly intellectual decay triggered by his current infatuated relationship with Mary—who was at least a worthy candidate for infatuation.

She was even amusing. And she knew when Sherlock lied. No one but Mycroft knew when Sherlock lied, and Mycroft could be sidetracked if you cleverly drew a more intriguing problem past that long beak of his. Sherlock had determined long since that a potential scandal in Parliament or a minor revolution in Central Asia could almost always draw Mycroft off the scent, so long as the lie wasn't too large or the key issues too irksome. Getting Mycroft to miss a lie about drug use? Nearly impossible. About a minor case? Quite easy, actually.

Anyway.

Best man. Right.

John had picked Sherlock. Not good old Mike Stamford. Not Lestrade. Not his old Army commander. Not any of dozens of choices.

He'd chosen Sherlock.

And Sherlock couldn't imagine why.

It wasn't the cases. You don't pick a man for having given you cases, or Lestrade or Mycroft would have won out—and would have to win out with Sherlock in that hypothetical marriage, too, which was most annoying of them. (He shooed an imagined Lestrade and Mycroft, both dapper in morning jackets and cumberbunds, to the lesser rooms of his Mind Palace…)

It wasn't loyalty—at least, not Sherlock's. It had taken time, but Sherlock did now understand that, from John's point of view, Sherlock had been complete rubbish in the loyalty department. And Sherlock didn't think John would have gone back to being loyal to him after all that if there weren't more there.

So…

Sherlock ticked off what he had learned about himself over long years of being told to "piss off" by many, many people, most of whom were willing to details his faults at length.

He was rude. A know-it-all. An ethical blank slate. A show-off. He was without empathy, patience, and such humor as he had apparently could be used in place of paint-stripper. He was self-centered, unforgiving, judgmental. His social skills were on a par with his modesty…non-existent.

It was a mystery.

Sherlock, then, stopped and drew that notion to the center of his mind palace, where it seemed to glow with a luster beyond pearls or diamonds.

John had chosen him as his best man. His best friend. The reason why was a mystery…

Sherlock sighed, then, and wrapped that wonderful little gift in his oft-neglected heart.

No matter what, no matter how long he lived, he'd always have one mystery to worry over on slow days, but never, ever solve.

John Watson had chosen him.

A better mystery he could not imagine.